Encoding method for converting multi-channel sound signals into 2-channel composite signals

ABSTRACT

An encoding method for use in a four-channel matrix system in which a front sound signal in one channel signal consists of a relatively large amplitude portion and a relatively small amplitude portion in phase quadrature with each other and the front sound signal in another channel signal is substantially in phase with the front sound signal in the one channel signal and consists of relatively small amplitude portions in phase quadrature with each other, and a back sound signal in one channel signal consists of a relatively large amplitude portion and a relatively small amplitude portion in phase quadrature with each other and the back sound signal in another channel signal is substantially in opposite phase with the back sound signal in the one channel signal and consists of relatively small amplitude portions in phase quadrature with each other. The present encoding method can improve the separation characteristic between the two composite signals and reduce the deterioration of sound quality particularly in two-channel reproduction of the two composite signals.

United States Patent 1191 Takahashi et al.

[ ENCODING METHOD FOR CONVERTING MULTI-CI-IANNEL SOUND SIGNALS INTO Z-CHANNEL COMPOSITE SIGNALS [75] Inventors: Susumu Takahashi; Ryosuke Ito,

both of Tokyo; Koichi Hirano, Chiba, all of Japan [73] Assignee: Sansui Electric Co., Ltd., Tokyo,

Japan 22 Filed: Dec. 26, 1973 211 App]. No.: 427,932

[30] Foreign Application Priority Data Sept 18, 1973 Japan 48-10517 [52] U.S. C1 179/1 GQ, 179/100.4 ST [51] Int. Cl. H04r 5/00 [58] Field of Search 179/1 GO. 1 G. 100.4 ST, 179/100.4 K

[56] References Cited UNITED STATES PATENTS 3,787,622 l/l974 ltoh et a1 179/1 GO 3,825,684 7/1974 Ito et a1 179/1 GQ OTHER PUBLICATIONS Journal of the Audio Engineering Society, Benjamin B. Bauer, Jan/Feb. 1973, Volume 21, Number 1, P. 19.

111] 3,872,249 1 1 Mar. 18, 1975 Audio, The Sansui Q S System, October 1971, P. 42.

Primary E.\'amint'rWilliam C. Cooper Assistant li.\'aminerTommy P. Chin Attorney, Agent, or FirmHarris, Kern, Wallcn & Tinsley [57] ABSTRACT An encoding method for use in a four-channel matrix system in which a front sound signal in one channel signal consists of a relatively large amplitude portion and a relatively small amplitude portion in phase quadrature with each other and the front sound signal in another channel signal is substantially in phase with the front sound signal in the one channel signal and consists of relatively small amplitude portions in phase quadrature with each other, and a back sound signal in one channel signal consists of a relatively large amplitude portion and a relatively small amplitude portion in phase quadrature with each other and the back sound signal in another channel signal is substantially in opposite phase with the back sound signal in the one channel signal and consists of relatively small amplitude portions in phase quadrature with each other. The present encoding method can improve the separation characteristic between the two composite signals and reduce the deterioration of sound quality particularly in two-channel reproduction of the two compos ite signals.

7 Claims, 9 Drawing Figures PHASE SillIlEli PHASI SHIFTER PATENTED I sew 1 OF 8 PATENTED "AR 1 8W5 sumaufg PAT TEUHARI8I975 sum u org FIG. 4A

FIG.4B

ENCODING METHOD FOR CONVERTING MULTI-CHANNEL SOUND SIGNALS INTO -C N EL .CQMPQ IG LS This invention relates to a 4-channel matrix encoding system for converting multi-channel sound signals into 5 Z-channel signals.

There is conventionally known an encoding system in which sound signals from sound sources located in front of the left-center and right'center of a sound field are respectively coupled with the same phase to a right and left channel transmission system and sound signals from sound sources positioned behind the left-center and right-center of the sound field are respectively coupled with an opposite phase to a right and left channel transmission system. In this case, it is preferred that a left-center signal be coupled only to a left transmission system and a righ-center signal be coupled only to a right transmission system.

As one example of the above-mentioned encoding system, there is known an encoding system adapted to synthesize a left channel signal L and right channel signal R which are shown in the following expression:-

in which A denotes a blend quantity whose representative value is 0.414 sin 22.5/cos 22.5").

As shown in the expression (1), the front-left signal I left signal L and back-left signal L included in the right channel signal R On the other hand, the front right signal R and back-right signal R included in the right channel signal R are greater in amplitude level than the front-right signal R and back-right signal R included in the left channel signal L The front signals L, and R included in the right and left channel signals L and R respectively, are phase shifted by a reference angle, and the back signals L and R included in the left channel signal L, are phaseshifted by an angle equal to a reference angle plus 90 while the back signals R and L included in the right channel signal R are phase shifted by an angle equal to a reference angle minus 90.

The encoding system as shown in the above expression (1) has the following advantages. Since the front signals are coupled with the same phase to the first and second channels and the back signals are coupled with an opposite phase to the first and second channels, 4- channel reproducing signals can be obtained merely by effecting the addition and subtraction of the first and second channel signals in a decoder device. This renders the decoder simpler in construction. Furthermore, even when a 4-channel matrix stereophonic record is reproduced using a stereophonic sound system, a clear image localization is obtained from the front signals. However, the encoding system suffers from the following disadvantages. With the encoder adapted to be sup.- plied with only four sound signals L R L and R,;, where a left-center signal L for example, is encoded, it will be sufficientif the abovementioned expression is given as L,-= L Thus, right and left signals L and R are represented as follows:

L1: Z V 2 LF +45 RT v As will be evident from the above expression, the amplitude ratio between the signals L and R is 12A with respect to the left-center signal L For this reason, a cross-talk (-7.7 db) of the quantity corresponding to a blend quantity A(=0.414) is produced at the right channel. The same is true with respect to the rightcenter signal R, and a cross-talk quantity (1-7.7 db) is produced at the left channel. This means that the basic object of transmitting the left-center signal and rightcenter signal respectively to one of the channels is not fully realized. With the above encoding system, since the sound signals L and R in the composite signals are blended with the same phase as will be clear from the expression (1), ambience components happening to be included in the sound signals L, and R, with an opposite phase are cancelled during the encoding process, thus presenting a problem. For this reason, when particularly a 4-channel matrix stereophonic record is reproduced using a conventional stereophonic reproduction system, a sound quality is deteriorated with the attendant disadvantage.

It is accordingly the object of this invention to provide a simple encoding system capable of reducing a cross-talk between the left and right channels with respect to the left-center signal and right-center signal and capable of reducing a sound quality deterioration even when reproducing is effected by the conventional stereophonic reproduction system.

According to the present invention there is provided an encoding method in which, in coupling to respective first and second channels at least first and second sound input signals associated with front channels and at least third and fourth sound input signals associated with back channels to generate first and second channel signals, said first, second, third and fourth sound input signals are coupled to the first and second channels with such an amplitude relation that the amplitude levels of said first and third sound input signals included in said first channel signal are greater than those of the first and third sound input signals included in the second channel signal, and the amplitude levels of the second and fourth sound input signals included in said second channel are greater than those of second and fourth sound input signals included in the first channel signal, and in such a phase relation that the first and second sound input signals included in said first channel signal are in a substantially in-phase relation to the first and second sound input signals included in said second channel signal respectively and the third and fourth sound input signals included in said first channel signal is in a substantially opposite relation to the third and fourth sound input signals included in said second channel signal respectively: said encoding method comprising the steps of coupling said first sound input signal to said first channel at a relatively large amplitude level and at a reference phase shift angle and to said first channel at a relatively small amplitude level and at a phase shift angle corresponding to the reference angle plus coupling said first sound input signal to said second channel at a relatively small amplitude level and at the reference phase shift angle and to said second channel at a relatively small amplitude level and at a phase shift angle corresponding to the reference angle plus 90; coupling said second input signal to said first channel at a relatively small amplitude level and at the reference phase shift angle and to said first channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90; coupling said second sound input signal to said second channel at a relatively large amplitude level and at the reference phase shift angle and to said second channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90; coupling said third sound input signal to said first channel at a relatively large amplitude level and at a phase shift angle corresponding to the reference angle plus 90 and to said first channel at a relatively small amplitude level and at the reference phase shift angle; coupling said third sound input signal to said second channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90 and to saidsecond channel at a relatively small amplitude level and at a phase shift angle displaced 180 fromsaid reference angle; coupling said fourth sound input signal to said first channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle plus 90 and to said first channel at a relatively small amplitude level and at the phase shift angle displaced 180 from said reference angle; and coupling said fourth sound input signal to said second channel at a relatively large amplitude level and at the phase shift angle corresponding to the reference angle minus 90 and to said second channel at a relatively small amplitude level and the reference phase shift angle.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a model diagram for explaining an encoding system according to this invention;

FIG. 2 shows vector diagrams of left and right channel signals L and R o'btainedby the encoding system of this invention with respect to respective input sound signals;

FIG. 3 shows vector diagrams of the left and right tion;

FIGS. 4A 4B show vector diagrams of the left and right channel signals, respectively,.of front-center and back center signals;

FIGS. 5 to 7 show vector diagrams of the left and right channel signals obtained under respective blend conditions; and

FIG. 8'shows a concrete example of the encoding system embodying this invention. FIG. 1 shows a model of an encoding system according to this invention. in the drawing reference numerals 1 to 4 denote encoding input terminals to which sound signals L L R and R are respectively applied, and reference numerals 5 and 6 represent encoding output terminals from which a left channel composite signal L and right channel composite signal R are respectively derived out. The sound signal L, is coupled to the output terminal 5 through a phase shifter 7 for phase shifting an input signal by a reference angle d) at a relatively large amplitude level and an adder 11. The sound signal R, has its amplitude multiplied by a blend quantity A 1) due to the action of a blend resistor R i.e. is coupled, at a relatively small amplitude level to the output terminal 5 through the phase shifter 7 and adder 11. The sound signal L has a relatively large amplitude channel signals obtained under a certain blend condilevel and is coupled to the output terminal 5 through a phase shifter 8 for phase shifting an input signal by an angle equal to the reference angle plus 90 and the adder 11. The sound signal R has its amplitude multiplied by a blend quantity A through a blend resistor R i.e. is coupled, at a relatively small amplitude level to the output terminal 5 through the phase shifter 8 and adder 1 l. The sound signal R, has a relatively large amplitude level and is coupled to the output terminal 6 through a phase shifter 10 having a phase shift characteristic similar to that of the phase shifter 7 and adder 12, while the sound signal L has its amplitude multiplied by a blend quantity A and is coupled to the output terminal6 through phase shifter 10 and adder l2.

' The sound signal R is coupled to the output terminal plitude multiplied by a blend quantity A, and is coupled to the output terminal 6 through phase shifter 9 and adder 12. Though the construction of the abovementioned encoder is the same as the conventional encoder, the encoding system .of this invention has blend quantites A and A in addition to the blend quantity A More particularly, the signal L is coupled to the out put terminal 5 through the phase shifter 8 with the blend quantity A due to a resistor R and also coupled to the output terminal 6 through an inverter 13 and the phase shifter 9 with the blend quantity A due to a resistor R The signal L is coupled through the phase shifter 7 to the output terminal 5 with the blend quantity A and also coupled to the output terminal 6 through an inverter 14 and phase shifter 10 with the blend quantity A due to a reisitor 5. The signal R is coupled through phase shifter 10 to the output terminal 6 with the blend quantity A and also coupled to the output terminal 5 through the inverter 13 and phase shifter 7 with the blend quantity A The signal R is coupled through phase shifter 9 to the output terminal 6 with the blend quantity A and also coupled through inverter 14 and phase shifter 8 to the output terminal 5 with the blend quantity A In the model diagram of FIG. 1 the inverters l3 and 14 are shown as of bidirectional type.

The matrix representation ofthe above-mentioned encoding system is given below:-

FIG. 2 shows vector diagrams of respective signals included in the right and left channel signals L and R obtained by the above-mentioned encoding system. The amplitude ratio of respective input signals contained in the out ut si nals L and R is. as will be clearly shown, I +A w/A, +A FIG. 3 shows the vector diagrams of the output signals L and R involved where A, A A 0.3063. In this case, the amplitude ratio of the respective input signals included in the output signals L and R is 1:0.414 as in the conventional system. With a left-center signal L (L, L,,) under this blend condition, as will be evident from the vector diagram of FIG. 3, the input signals L and L in the output signal R are cancelled with respect to each other with the result that no left-center signal L appears in the output signal R With a right-center signal R (R; R the input signals R and R in the output signal L are cancelled with respect to each other. As a result, no right-center signal R appears in the output signal L More particularly the left-center signal L is transmitted only to a left channel transmission system and the right-center signal R is transmitted only to a right channel transmission system. Unlike the conventional encoding system, therefore, no cross-talk exists between left and right channel signals with respect to left and right-center signals.

FIGS. 4A and 4B show vector diagrams of the output signals L and R respectively, involved when the front-center signal C (L R and bac k-center signal C (L R are encoded by the encoding system as described above. Like the conventional system, in the case ofthe front-center signal C the output signals L and R have the same phase and the same level, while in the case of the back-center signal C the output signals L and R have opposite phases and the same level. From the above it will be understood that the encoding system of this invention is suitable for use as a 4- channel matrix encoding system.

As will be understood from the vector diagram of FIG. 3, a phase difference of 27.98 exists between each of the front input signals Ly and R in the output signals L and R while a phase difference of I52.02 exists between each of the rear input signals L and R in the output signals L and R Therefore, separation between diagonal channels is not made infinite and stays at 15.2 db. However, with the left and right-center signals, separation between the output signals L and R is made infinite and in the case of the front-center signal C the output signals L and R have the same phase. As a result, the output signals obtained by the above-mentioned encoding system represent a particularly good compatibility with the conventional twochannel stereophonic reproduction system. A phase difference of 28 present between the corresponding front input signals included in the above-mentioned output signals L and R may be safely taken as meaning that the respective front input signals are coupled, in substantially the same phase relation, to the left and right transmission systems. A phase difference of 152 present between the corresponding back signals may be safely taken as meaning that the respective input signals are coupled, in a substantially opposite phase relation, to the left and right transmission systems.

Where the blend quantities A A andA are all equal to 0.3063, the respective front input signals included in the output signals L and R do not become entirely the same phase and the respective back input signals do not become entirely an opposite phase. In order to cause the respective front input signals to be entirely in phase and the respective back input signals to be entirely in opposite phase, it is only necessary to proportion the blend quantities to have a relation of 12A, A zA FIG. 5 shows a vector diagram of the output signals L and R involved when the blend quantities are selected to be A,=A-,=0.4I42 and A =0.l7l5 so as to meet the relation. As will be evident, the corresponding front input signals included in the output signals are caused to be entirely in phase and the corresponding back input signals to be entirely in opposite phase. Under this blend condition, with regard to the front center signal C; a

phase difference of about 19 exists between the output signals L and R and, therefore, the sound image localization of the front center signal becomes somewhat indistinct during 2-channel stereophonic reproduction. With regard to the left and right-center signals 1 and R separation between the output signals L and R stays at 15.3 db. Generally, however, this separation is sufficient from the practical view point.

FIG. 6 shows the vector diagrams of the output Sig nals L and R involved when A =0.37 l4 and A =A 0.2. Under this blend condition, with regard to the front-center signal C the output signals L and R are in phase and a phase difference between the front input signals is held down to 17. With respect to the left and right-center signals there is obtained separation of 16 db between output signals L and R and separation between the diagonal channels is 19.57 db.

FIG. 7 shows a vector diagram of the output signals L and R involved when A =0.383, A =0.330 and A 0.2082. Under the blend condition thus far described, with regard to the respective front input signals or front-center signals, encoding is effected to cause the output signals L and R to be in phase. When with regard to the respective front input signals ihoutpm'si'g nals L and R are caused to be in phase, a phase difference between thee output signals becomes greater with respect to the frontcenter signal. When with regard to the front-center signal the output signals are caused to be in phase, a phase difference between the output signals becomes greater with respect to the respective front input signals. In the case of FIG. 7, with regard to either the respective front input signals or the frontcenter input signal, a phase difference between the output signals L and R can be held down to about 10. More particularly a phase difference between the respective front input signals is l0.27 and a phase difference between the front-center input signals is 10.07. In this case, left and right separation with respect to left and right-center signals L and R is about 17 db and a better compatibility is obtained with regard to a 2- channel stereophonic reproduction. Furthermore, separation between the diagonal channels is 25.35 db.

With the encoding system according to this invention, the blend quantity A, may be varied between the front and back directions; the blend quantity A; between the left and right directions and the blend quantity A between the two diagonal directions. According to this invention, the blend quantities A A and A may take any value below unity.

FIG. 8 shows one example of an encoding system according to this invention in which elements identical to those shown in FIG. 1 are designated by the same reference characters, so that any detailed explanation is believed unnecessary. In FIG. 8, shifting by til-90 under the action of a phase shifter 9 the phase of a signal produced by inverting a signal L for example, under the action of an inverter 13 is equivalent to shift of the phase of the signal L by +90 without inverting this signal, and shifting by a reference angle under the action of a phase shifter 7 the phase of a signal produced by inverting a signal L), under the action of an inverter 14 is equivalent to shift of the phase of signal L by d il without inverting the signal.

With the encoding system according to this invention, it is possible to optionally vary a phase difference between the respective input signals by varying the blend condition, and therefore it is possible to produce left and right channel signals having a compatibility practically sufficient for the conventional Z-channel stereophonic reproduction in respect of separation between the left and right channels, image localization involved in the case of front-center input signals and sound quality. Thus, the encoding system can be of simple construction because phase shifters as used in the conventional encoding system can be used intact.

What we claim is:

1. An encoding method in which, in coupling to respective first and second channels at least first and second sound input signals associated with front channels and at least third and fourth sound input signals associated with back channels to generate first and second channel signals, said first, second, third, and fourth sound input signals are coupled to the first and second channels with such an amplitude relation that the amplitude levels of said first and third sound input signals included in said first channel signal are greater than those of the first and third sound input signals included in the second channel signal and the amplitude levels of the second and fourth sound input signals included in said second channel are greater than those of second and fourth sound input signals included in the first channel signal, and in such a phase relation that the first and second sound input signals included in said first channel signal are in a substantially in-phase relation to the first and second sound input signals included in said second channel signal respectively and the third and fourth sound input signals included in said first channel signal is in a substantially opposite relation to the third and fourth sound input signals included in said second channel signal respectively: said encoding method comprising the steps of coupling said first sound input signal to said first channel at a relatively large amplitude level and at a reference phase shift angle and to said first channel at a relatively small amplitude level and at a phase shift angle corresponding to the reference angle plus 90; coupling said first sound input signal to said second channel at a relatively small amplitude level and at the reference phase shift angle and to said second channel at a relatively small amplitude level and at a phase shift angle corresponding to the reference angle plus 90; coupling said second input signal to said first channel at a relatively small amplitude level and at the reference phase shift angle and to said first channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90; coupling said second sound input signal to said second channel ata relatively large amplitude level and at the reference phase shift angle and to said second channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90; coupling said third sound input signal to said first channel at a relatively large amplitude level and at a phase shift angle corresponding to the reference angle plus 90 and to said first channel at a relatively small amplitude level and at the reference phase shift angle; coupling said third sound input signal to said second channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90 and to said second channel at a relatively small amplitude level and at a phase shift angle displaced 180 from said reference angle; coupling said fourth sound input signal to said first channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle plus 90 and to said first channel at a relatively small amplitude level and at the phase shift angle displaced 180 from said reference angle; and coupling said fourth sound input signal to said second channel at a relatively large amplitude level and at the phase shift angle corresponding to the reference angle minus 90 and to said second channel at a relatively small amplitude level and the reference phase shift angle.

2. An encoding method according to claim 1 in which said first and second channel signals L and R are represented by the following matrix third and fourth sound input signals, respectively; and A A and A represent coefficients each having a value less than unity.

3. An encoding method according to claim 2 in which said coefficients have a relation of A, A A

4. An encoding method according to claim 3 in which said coefficients have a relation of A A A about 0.3.

5. An encoding method according to claim 2 in which said coefficients have a relation of lzA A :A

6. An encoding method according to claim 5 in which said coefficients A and A each have a value of about 0.41 and said coefficient A has a value of about 0.17.

7. An encoding method according to claim 2 in which said coefficient A has a value of about 0.37 and said coefficients A and A each have a value of about 0.2.

l l l l 

1. An encoding method in which, in coupling to respective first and second channels at least first and second sound input signals associated with front channels and at least third and fourth sound input signals associated with back channels to generate first and second channel signals, said first, second, third, and fourth sound input signals are coupled to the first and second channels with such an amplitude relation that the amplitude levels of said first and third sound input signals included in said first channel signal are greater than those of the first and third sound input signals included in the second channel signal and the amplitude levels of the second and fourth sound input signals included in said second channel are greater than those of second and fourth sound input signals included in the first channel signal, and in such a phase relation that the first and second sound input signals included in said first channel signal are in a substantially in-phase relation to the first and second sound input signals included in said second channel signal respectively and the third and fourth sound input signals included in said first channel signal is in a substantially opposite relation to the third and fourth sound input signals included in said second channel signal respectively: said encoding method comprising the steps of coupling said first sound input signal to said first channel at a relatively large amplitude level and at a reference phase shift angle and to said first channel at a relatively small amplitude level and at a phase shift angle corresponding to the reference angle plus 90*; coupling said first sound input signal to said second channel at a relatively small amplitude level and at the reference phase shift angle and to said second channel at a relatively small amplitude level and at a phase shift angle corresponding to the reference angle plus 90*; coupling said second input signal to said first channel at a relatively small amplitude level and at the reference phase shift angle and to said first channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90*; coupling said second sound input signal to said second channel at a relatively large amplitude level and at the reference phase shift angle and to said second channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90*; coupling said third sound input signal to said first channel at a relatively large amplitude level and at a phase shift angle corresponding to the reference angle plus 90* and to said first channel at a relatively small amplitude level and at the reference phase shiFt angle; coupling said third sound input signal to said second channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle minus 90* and to said second channel at a relatively small amplitude level and at a phase shift angle displaced 180* from said reference angle; coupling said fourth sound input signal to said first channel at a relatively small amplitude level and at the phase shift angle corresponding to the reference angle plus 90* and to said first channel at a relatively small amplitude level and at the phase shift angle displaced 180* from said reference angle; and coupling said fourth sound input signal to said second channel at a relatively large amplitude level and at the phase shift angle corresponding to the reference angle minus 90* and to said second channel at a relatively small amplitude level and the reference phase shift angle.
 2. An encoding method according to claim 1 in which said first and second channel signals LT and RT are represented by the following matrix
 3. An encoding method according to claim 2 in which said coefficients have a relation of Delta 1 Delta 2 Delta
 3. 4. An encoding method according to claim 3 in which said coefficients have a relation of Delta 1 Delta 2 Delta 3 about 0.3.
 5. An encoding method according to claim 2 in which said coefficients have a relation of 1: Delta 2 Delta 1: Delta
 3. 6. An encoding method according to claim 5 in which said coefficients Delta 1 and Delta 2 each have a value of about 0.41 and said coefficient Delta 3 has a value of about 0.17.
 7. An encoding method according to claim 2 in which said coefficient Delta 1 has a value of about 0.37 and said coefficients Delta 2 and Delta 3 each have a value of about 0.2. 